1. Field of the Invention
The present invention relates to magnesia-based beta alumina sintered bodies and a process for producing the same. Such a magnesia-based beta alumina sintered body is utilized as a solid electrolyte to be used in sodium-sulfur cells.
The beta alumina has four kinds of crystal systems: .beta.-alumina, .beta."-alumina, .beta.'"-alumina, and .beta.""-alumina. .beta.-Alumina and .beta."-alumina are mainly precipitated by ordinary producing processes. In the specification and claims of this application, the beta alumina sintered body is composed mainly of .beta.-alumina and .beta."-alumina. The term of the "magnesia-based" beta alumina sintered body means a beta alumina sintered body in which magnesia is used as a stabilizer.
2. Related Art Statement
The beta alumina sintered bodies have been heretofore used as the solid electrolytes of the sodium-sulfur cells. From the standpoint of performance, service life and operation reliability of the cell, the solid electrolyte is required to have large mechanical strength and low electric resistance.
It has been formerly known that beta alumina having high strength and low electric resistance can be obtained by controlling the contents, ratios and crystalline particle diameters of .beta.-alumina crystals and .beta."-alumina crystals. For example, "Journals of MATERIALS Science 19 (1984), pp 695-715" describes that the electric resistance varies with the ratios of the .beta.-alumina crystals and .beta."-alumina crystals, and that the larger the .beta."-alumina, the lower is the electric resistance (See p 703, FIG. 12).
GB-B-1558305 discloses a process for producing a beta alumina sintered body having high electric conductivity, a fine crystalline structure and high durability. In this patent, having noted that it is difficult to obtain a uniform crystalline structure having high electric conductivity due to densification occurring in a single heating/cooling cycle when firing is effected along a single heating/cooling curve as done conventionally, a beta alumina sintered body having high electric conductivity and a uniform crystalline structure is obtained by repeating heating/cooling twice or more such that not more than 95% of the overall linear shrinkage takes place during any one cycle of heating/cooling.
Further, U.S.P. discloses a process for producing a beta alumina sintered body having high strength and low electric resistance. In this pores, special starting compounds composed alumina, a sodium compound and a lithium-aluminum compound (Li.sub.2 O.nAl.sub.2 O.sub.3) are used, and a beta alumina sintered body is obtained by firing at 1500.degree.-1600.degree. C. for a short time period such as less than 10 minutes.
However, the above Journals of MATERIALS Science discloses a process for producing the beta alumina sintered body having low electric resistance, but has no mentioning about a process for producing a beta alumina sintered body having low resistance and high strength.
On the other hand, the two-stage peak firing process in GB-B-1558305 has various problems in practical application. That is, while the temperature distribution widely varies inside a large-scale furnace for mass production, it is necessary to extremely sharply vary the temperature distribution inside the furnace so that heating/cooling may be repeated along a given heat curve with the lapse of time. However, it is difficult to control the temperature distribution inside the furnace in such a manner particularly in the case of the mass production type large-scale furnace. As a result, it was extremely difficult to mass produce products having less variation in quality at a high yield.
On the other hand, U.S. Pat. No. 4,113,928 describes the process for producing the lithia-based beta alumina sintered bodies, but does not specifically disclose anything about a process for producing magnesia-based beta alumina. In this process for producing the lithia-based beta alumina sintered bodies, the firing is effected at 1500.degree.-1600.degree. C., and the molded body is kept at this firing temperature for an extremely short period, e.g., less than 10 minutes. Therefore, this process has also various problems in the practical application. That is, since the temperature distribution widely varies in the large-scale furnace for mass production, it takes a long time to make a temperature in a lower temperature zone follow that in a higher temperature zone. Therefore, the short firing temperature requires that even a large-scale furnace has an extremely excellent temperature distribution. However, it is difficult to produce such a large-scale furnace.
Therefore, the above processes are inappropriate as a producing process for the mass production of the beta alumina sintered bodies.